An inorganic membrane may be applied, for example, as a porous coating on a porous ceramic support. Inorganic membranes offer several advantages over organic membranes. Inorganic membranes, for example, typically have high chemical and thermal stabilities that allow the membranes to be used in extreme pH and chemical environments. In addition, inorganic membranes can be easily cleaned by applying high temperature treatments such as firing.
Inorganic membranes may be used for filtration and separation applications in the environmental, biological, food and drink, semiconductor, chemical, petrochemical, gas and energy industries. These industries often require purified gas/vapor or purified liquid whose source is a mixed feed stream composed of different gas and/or liquid/particulate combinations. Specific examples include purification and separation of hydrogen gas, sequestration of carbon dioxide gas, filtration of oil/water mixtures, wastewater treatment, filtration of wines and juices, filtration of bacteria and viruses from fluid streams, separation of ethanol from biomass, and production of high purity gas and water for the semiconductor and microelectronics industry.
Inorganic membranes may be applied as layered structures comprising a porous inorganic monolayer or multilayer coating on a porous support, such as a ceramic support. The porous coating layer is generally prepared by dipping the support into a coating slip and by subsequently withdrawing it out of that slip, followed by drying and firing.
The coating slip is a dispersion of solid particles in a liquid. Fine particles that are in the colloidal range (≦1 um) usually aggregate in the dispersing medium due to relatively high strength of the inter-particle van der Waals attractive forces. Thus, a dispersant such as Darvan C, Tiron or Aluminon is often introduced to build up a repulsive force barrier and to stabilize the slips (Briscoe, Khan, Luckham, J. Europ. Ceram. Soc., 18 (1998) 2141-2147). Also, coating slips generally contain more than one polymeric compound, such as surfactants, lubricants, and plasticizers. The interaction between all these compounds determines the slip behavior and the microstructure development during compaction, drying and calcining (Burggraaf and Cot, Fundamentals of Inorganic Membrane Science and Technology, Elsevier Science B.V., 1996, Page 157).
WO 85/01937 discusses a process to make adherent Microfiltration coatings on the inside of macroporous tubes by filling and draining a tube with a de-agglomerated alumina slip and subsequent drying and firing of the coating. Besides 8 wt. % alumina powder, the slip contained polyethylene glycol (PEG) and Darvan C dispersant by 0.2%. The slip was ball-milled for 24 h, which was believed to be essential for breaking up the agglomerates and dispersing the particles well.
EP 0344961 B1 discusses another formulation of an inorganic coating slip used for coating on a porous metal. The slip comprises 60-95% by weight of relatively larger inorganic particles such as alumina and zirconia, the balance being of much smaller particles. The larger particles may have an average in the range of 0.5-50 um, chosen to generate membranes with pores of desired size. The smaller particles may have an average size of 4 nm up to 1 um, but not more than 0.1 times the size of the larger particles. The smaller particles act as a sintering aid, enabling the membrane to be sintered at lower temperatures. The portions of the smaller particles should not be too large to substantially block the pores between the larger particles.
Inorganic membrane coatings furthermore often encounter problems of mud cracking, de-lamination, and pore closure. Some anti cracking organic materials, such as, PEG, PVP, PVA, etc, are often used in the coating slip. However, in many cases, those additives are not effective. Another problem encountered with inorganic membrane coating relates to pore structure. For high flux, uniform pore structures and large porosity is desired. However, in conventional coating processes, pore structures are generally formed from particle packing during drying and firing process, thus limiting porosity.
In view of the above, there is a need in the art for more favorable processes for depositing porous membranes of inorganic particles on porous supports.